Metal halide discharge lamp having a thermally insulative end coating

ABSTRACT

The ends of an unjacketed metal halide arc discharge lamp are covered with a thermally insulative coating in order to reduce the rate of cooling thereof after lamp shut-off.

United States Patent 1 Muzeroll 5] Nov. 4, 1975 [5 METAL HALIDE DISCHARGE LAMP 2,087,735 7/1937 Pirani et a1. H 313/47 x H VI A THERMALLY INSULATIVE END 2,215,648 9/1940 Marden et a1. 313/27 COATING 3,333,132 7/1967 Edris et a1 H 313/47 X 3,619,683 11/1971 Weston et al 313/47 [75] Inventor: Martin E. Muzeroll, Manchester, 3,753,019 8/1973 Hellman 313/47 [73] Assignee: GTE Sylvania Incorporated, primary Examiner |ames B Munins Danvers, Mass Attorney, Agent, or Firm-James Theodosopoulos [22] Filed: Mar. 22, 1974 [21] Appl. No: 453,695

[57] ABSTRACT 2% 313/43; 313/44; 313/47 The ends of an unjacketed metal halide arc discharge E i lamp are covered with a thermally insulative coating 0 Seam in order to reduce the rate of cooling thereof after [56] References Cited lamp UNITED STATES PATENTS 1 Claim, 1 Drawing Figure 2,042,261 5/1936 Krefil ,1 313/47 X US. Patent Nov. 4, 1975 METAL HALlDE DISCHARGE LAMP HAVING A THERMALLY INSULATIVE END COATING THE INVENTION Tubular high intensity metal halide arc discharge lamps are made in various shapes and sizes and usually contain, in addition to mercury and an inert starting gas, a metal additive, usually as the halide, such as galliurn, thallium, scandium, indium, sodium and the like. These lamps are often used in reprographic equipment and for some types of reprographic equipment it is expedient to mount the lamps vertically.

The lamps are made of high temperature glass, such as quartz, have electrodes at each end of the lamp and are sealed at each end by glass to metal seals.

When vertically operated lamps are de-energized, the metallic additive condenses in the lower seal area which is the part of the lamp that cools first, Repeated vaporizing and condensing of the metallic additive in the seal area causes a cleavage in the quartz at the seal.

The purpose of this invention is to greatly reduce early seal failures caused by the metallic additive condensing at the seal. This reduction in seal failure is ac complished by the application of thermally insulative coating to the quartz envelope at the seal areas. The coating prevents the seal area from cooling as rapidly as other areas of the lamp, thereby decreasing the condensation of additives in the sea] area.

it is known, in the field of metal halide arc discharge lamps, to use heat reflective coatings at the ends of an arc tube in order that said ends be maintained sufficiently hot during normal lamp operation so as to pro vide an effective vapor pressure of the metallic additives within the arc tube; see. for example, U.S. Pat. Nos. 3,325,662 and 3,374,377. Such coatings normally comprise a powdered material and are usually quite thin, since only heat reflection is required. The adhesion of such coatings is usually quite weak but, since the arc tube is enclosed within an outer glass jacket, little rubbing off occurs.

The thermally insulative coating of this invention is much thicker than the heat reflective coatings of the prior art, since its function is to prevent the ends of the arc tube from cooling off more rapidly than the body of the arc tube, after the lamp is extinguished. ln contrast, the function of said heat reflective coatings is to reflect heat only during lamp operation; they are generally too thin to provide adequate thermal insulation after lamp extinguishment. it is also noted that are tube end cooling is less severe in jacketed lamps.

Since lamps having a thermally insulative coating in accordance with this invention are generally used with out an outer jacket, the coating comprises an inorganic fibrous material to provide buildup and good adhesion to the arc tube in order to resist rub-off.

The single FIGURE in the drawing is an elevational view, partly in section, of a metal halide arc discharge lamp in accordance with this invention.

The drawing shows a U-shaped tubular lamp which is mounted with the two bases down. The lamp is composed of a U-shaped quartz envelope 1 with electrodes 2 at each end attached by means of rods 7 to thin pieces of molybdenum 3 press sealed into the quartz envelope in a conventional manner. The outer ends of the molybdenum are attached to external leads 4. Bases 5 are cemerited to the lower part of the pressed seals. The ends of envelope 1 are covered with a thermally insulative 2 coating 6. Preferably, coating 6 covers the press seal portions of the arc tube and extends up to. or slightly beyond, electrode When electrode 2 and molybdenum foil 3 are pressed into the quartz, the quartz does not make a perfect seal around electrode rod 7. There is a small space along part of the rod as it enters the pressed quartz. When the lamp is de-energized and starts to cool, the additives in the lamp start to condense at the coolest part of the lamp which is the seal area. in the process of condens ing, some of the metallic additive will collect in the space between the electrode rode and the quartz When the lamp is energized, some of the additive will remain. Repeated vaporizing and condensing will cause more of the additive to collect.

The difference in thermal characteristics of the metal additive and the quartz causes a strain on the quartz. and in time causes a cleavage in the quartz. This cleav age permits the metallic additive to work its way closer to the molybdenum foil seal. When the metallic addi tive reaches the molybdenum foil it erodes the foil and deteriorates the seal between the foil and the quartz. Once the seal is completely deteriorated. the lamp fails.

Coating the quartz around the lamp seal areas keeps these areas from cooling as rapidly and the additives will tend to condense in other areas of the lamp. Reducing the condensation of the metallic additives in the sea] area reduces seal failures and greatly increases the life of vertically operated tubular lamps.

In one embodiment of this invention, envelope 1 consisted of a U-shaped hali inch diameter quartz tube containing a filling of argon, mercury, gallium and men curic iodide. The lamp was rated for 300 hours life at 1500 watts operation and had an arc length of8 inches.

Prior to application coating 6, the ends of envelope 'l, that is, the well portions surrounding electrodes 2 were platinized. The purpose of this thin metallic platinum. coating was to provide heat reflectivity, for the same reason as the prior art heat reflective coatings prc i" ously mentioned.

Next, coating 6 was applied by brush over the metal reflective coating and extended onto the press seals at least as far as the weld of rod 7 to molybdenum ribbon 3. Coating 6, applied as a liquid dispersion, was essentially a ceramic coating and comprised ball-milled alu minum silicate fibers bonded with inorganic addit'unuv such as phosphates. It was air setting and dried to hard surface resistant to abrasion. The thickness of coating 6 was about 15 mm.

These lamps, containing thermally insulative and coating 6, were compared with identical lamps but without the thermally insulative coating. The Ctitliput'i' son was made by operating the lamps in a vertical nsi tion for 3 cycles per day, each cycle consisting oi 7 hours on and l hour off. During the on time, the lamp. were additionally continuously cycled to operated for 5 minute at i000 watts and 1/2 minute at 1500 watts.

The uncoated lamps all failed at a total of 33 ltlnulrs In 1500 watt operation, the average life being 25 hours. in contrast, the first failure of a coated lamp occurred at ll3 hours; even after 167 hours, of the oated lamps were still in normal operating condition.

The seal temperatures of the lamps are about 42131. at [500 watt operation and about 370C at watt operation. The are tube wall temperature is about 800C at i500 watt operation and about 750C at lliilll watt operation.

4 a glass-to-metal seal and being covered with a thin metal reflective coating which is overcoated with a bonded thermally insulative coating, the thickness of said insulative coating being such that, during vertical lamp operation with both ends of the U downward and upon cxtinguishment of the lamp, said insulative coating maintains sufficient heat in said ends to prevent condensation of said fill at said glass-to-metal seal of both of said ends.

I! it t k 

1. AN UNJACKETED METAL HALIDE ARC DISCHARGE LAMP COMPRISING A U-SHAPED GLASS ARC TUBE HAVING A FILL INCLUDING INERT STARTING GAS, NERCURY AND METAL HALIDE AND CONTAINING AN ELECTRODE AT EACH END OF THE U, THE ENDS OF SAID U-SHAPED ARC TUBE BEING SEALED AND CONTAINING A GLASS-TO-METAL SEAL AND BEING COVERED WITH A THIN METAL REFLECTIVE COATING WHICH IS OVERCOATED WITH A BONDED THERMALLY INSULATIVE COATING THE THICKNES OF SAID INSULATIVE COATING BEING SUCH THAT, DURING VERTICAL LAMP OPERATION WITH BOTH ENDS OF THE U DOWNWARD AND UPON EXTINGUISHMENT OF THE LAMP. SAID INSULATIVE COATING MAINTAINS SUFFICIENT HEAT IN SAID ENDS TO PREVENT CONDENSATION OF SAID FILL AT SAID GLASS-TO-METAL SEAL OF BOTH OF SAID ENDS. 